Such a flexible armor is for example produced according to the normative documents API 17J (Specification for Unbonded Flexible Pipe) and API RP 17B (Recommended Practice for Flexible Pipe) established by the American Petroleum Institute.
The pipe is generally formed with a set of concentric and superposed layers. It is considered as «unbounded» in the sense of the present invention and from the moment that at least one of the layers of the pipe is able to move longitudinally relatively to the adjacent layers during flexure of the pipe. In particular, an unbonded pipe is a pipe without any binding materials connecting layers forming the pipe.
The pipe is generally positioned through an extent of water, between a bottom assembly, intended to collect the exploited fluid in the bottom of the extent of water and a floating or fixed surface assembly, intended to collect and distribute the fluid. The surface assembly may be a semi-submersible platform, a FPSO or another floating assembly.
In certain cases, for exploiting fluids in deep waters, the flexible pipe has a length of more than 800 m. The ends of the pipe have endpieces for connecting to the bottom assembly and to the surface assembly, as well as for the intermediate connections.
These pipes are subject to very strong forces in axial traction, notably when the extent of water in which is positioned the pipe is very deep.
In this case, the upper endpiece connecting the pipe to the surface assembly should absorb a very large axial tension, which may attain several hundred tons. These forces are transmitted to the endpiece via the tensile armor layers extending along the pipe.
The axial tension not only has a high average value, but also permanent variations depending on the vertical movements of the surface assembly and of the pipe, under the effect of agitation of the extent of water caused by the swell or by waves.
The variations of axial tension may attain several tens of tons and be continually repeated during the service time of the pipe. Within 20 years, the number of cycles may thus attain more than 20 million.
Therefore it is necessary to ensure a particularly robust attachment between the tensile armor layers and the body of the endpiece.
For this purpose, in known end pieces, the anchoring of the armors is generally ensured by friction between the armor wires and the epoxy resin cast into the chamber delimited by the vault and the cover.
Moreover, the capstan effect related to the helical trajectory of the armor wires also contributes to anchoring of the armors, this effect may be increased by modifying the diameter of the helix described by the wires in the endpiece relatively to the diameter of this helix in current length, for example by gradually increasing this diameter following an ascending cone, and then by reducing it along a descending cone.
Further, deformations with the shape of a hook, or of a wave or of a twist may be formed at the end of each armor wire so as to be engaged into the epoxy resin, in order to produce mechanical blocking opposing the applied tension. These deformations initiate the force required for setting the capstan effect into place.
Such an endpiece does not give entire satisfaction. Sometimes, over time, the anchoring of the tensile armors becomes faulty by fatigue.
The failure may occur in the rear portion, at the area of detachment of the armors relatively to the diameter of the current length, this portion being further weakened during the mounting because of folding and unfolding the armors, required for setting into place the endpiece.
In order to overcome this problem, WO 2013/074098 describes a flexible pipe endpiece, in which the crimping of the internal sheath is carried out from the front, in order to avoid deformation of the armor elements which consist here in a composite material.
The end segments of the armor elements adopt a divergent conformation from the rear to the front during the setting into place of the endpiece, and retain this conformation in the endpiece.
Such an endpiece does not give entire satisfaction. Indeed, the conformation of the end segments places them in contact with each other in the chamber for receiving the resin.
The contact surface between each end segment and the resin is therefore reduced relatively to a more traditional structural endpiece, which reduces the fatigue strength of the endpiece, or which requires a considerable elongation of the endpiece for maintaining its absorption of axial tension.